1. Technical Field
This invention relates to the estimation of a signal with a physical time delay, and more particularly to a system of reducing or compensating for the time delay caused by a physical impediment in a signal using an infinite impulse response (“IIR”) filter.
2. Background Information
Signals from sensors for a variety of physical phenomena (such as pressure, temperature, flow, acceleration, heat flux, and optical intensity) may be delayed by a physical impediment. For example, in order to measure the temperature of a process fluid flowing through a conduit, a temperature sensor may be positioned in the fluid flow. However, it is often necessary to physically separate the temperature sensor from the fluid flow, e.g., due to compatibility issues. For example, the process fluid to be measured may be chemically incompatible with metallic temperature sensors, e.g., resulting in chemical attack or contamination of the solution and/or electrodes. In addition, the fluid may damage the temperature sensor, and build up of process fluid on the sensor may decrease the sensor's sensitivity. The fluid may also be part of a sanitary process, in which foreign objects such as sensors should not contact the process fluid. These issues may thus tend to preclude the placement of conventional temperature detectors in direct contact with the process fluid.
A conventional approach is to place the temperature sensor within a protective casing. With such a casing, the temperature sensor may be placed within the process fluid flow, while being protected from the process fluid by the casing. This approach relies on thermal conduction through the casing wall to the temperature sensor, to obtain temperature data. A drawback of this traditional solution is that the casing acts as a temperature insulator, thus impeding the sensor's ability to detect temperature change.
In typical examples, casings for temperature detectors to be placed in conduits containing corrosive fluids are fabricated from polymers such as PFA (perfluoroalkoxy polymer resin), PTFE (polytetrafluoroethylene), polyvinyl chloride (PVC), or various combinations thereof, such as perfluoroalkoxy-polytetrafluoroethylene co-polymer. The relatively poor thermal conductivity of these materials tends to adversely affect the accuracy and response time provided by such external temperature detection approaches. Some techniques for compensating for the inaccuracy and delay of such temperature sensors involve the use of additional temperature sensors, including sensors positioned on the outside of the conduit for the process fluid flow. Differences between the signals captured from these multiple sensors may be used to help estimate or otherwise compensate for the time delay. These techniques, however, may be impractical for many applications, such as those involving relatively complicated, expensive casings, such as those which may contain other devices in addition to sensors. It may thus be cost prohibitive to use multiples of these relatively expensive, complicated casings on the conduit.
Referring to the chart of FIG. 1A, the temperature 23 detected by a conventional temperature sensor in a polymer casing is plotted relative to actual process fluid temperature 21. As can be seen from the chart, there is a time lag between the process fluid temperature 21 and the detection of the temperature 23 by the sensor. In addition, the detected temperature 23 has a relatively flattened amplitude and fails to reach the highs and lows of the process temperature 21.
Turning to FIG. 1B, one attempt to overcome these drawbacks includes passing a signal 7, generated by the temperature sensor 3 within casing 5, through a conventional filter 9. Conventional filter 9 is used to process the signal to reduce noise (e.g., electrical interference from electronic hardware), such as by using averaging techniques to filter out electronic noise and to output a conventionally estimated temperature 11.
As can be seen in FIG. 1C, this conventionally filtered signal 25, while it may tend to reduce signal noise, tends not to compensate for the time lag. Rather, the conventionally filtered signal 25 may be viewed as increasing the time lag.
A need therefore exists for a system that compensates for, or otherwise mitigates the effect of time-related impediments to accurate quality measurements, without the need for multiple sensors.